Dynamic laser diode compensation

Abstract

A laser drive circuit compensates for laser diode dynamics. A compensation value is determined from a sum of weighted basis functions. The basis functions may be a function of current desired optical powers and/or past desired optical powers. The weights may be updated periodically based at least in part on accumulated basis function outputs and measured optical powers.

What is claimed is: 
     
       1. An apparatus comprising:
 a laser diode that exhibits a time varying nonlinear operating characteristic based at least upon past drive currents; 
 a plurality of circuits to calculate basis function outputs as a function of desired optical power values, wherein the basis functions are chosen to provide compensation for the time varying nonlinear operating characteristic; 
 a plurality of multipliers to weight the basis function outputs, and a first summer to create a compensation value from the multiplier outputs; 
 a second summer to add the compensation value to the desired optical power value to create a compensated desired optical power value; 
 an inverse laser model circuit to map compensated desired optical power value to a drive value; and 
 a digital-to-analog converter and drive circuit to receive the drive value and to drive the laser diode. 
 
     
     
       2. The apparatus of  claim 1  further comprising:
 a photodiode to measure optical power produced by the laser diode; 
 an integrator to integrate the output of the photodiode; 
 an analog-to-digital converter to digitize an output of the integrator to create {circumflex over (M)}; and 
 an accumulator to accumulate desired optical power values to create M. 
 
     
     
       3. The apparatus of  claim 2  wherein the desired optical power values represent pixels in an image, and the integrator integrates over one line of pixels in the image. 
     
     
       4. The apparatus of  claim 2  further comprising a processing system to modify weights as a function of M, {circumflex over (M)}, and past weights. 
     
     
       5. The apparatus of  claim 4  wherein the processing system calculates the weights as:
       n+1 =(1−α)   0 +α   n +β(μ w +( C   w   −1   +A   T   C   noise   −1   A ) −1   A   T   C   noise   −1 ( M−{circumflex over (M)}−Aμ   w ))
 
 where: 
     n+1  are the new weights, 
     n  are the existing weights, 
 μ w  is 
 A are the accumulated basis function outputs, 
 M are the accumulated desired optical power values, 
 {circumflex over (M)} are the integrated measured optical power values, 
 C w  is the basis function covariance matrix, 
 C noise  is the measured noise covariance matrix, 
     0  are the initial weights, 
 α is a restoring constant (default is 1), and 
 β is a learning constant (default is 1). 
 
     
     
       6. The apparatus of  claim 1  wherein the basis function outputs are at least a function of a present desired optical power value. 
     
     
       7. The apparatus of  claim 1  wherein the basis function outputs are at least a function of a past desired optical power value. 
     
     
       8. The apparatus of  claim 1  wherein the basis function outputs are a function of both present and past desired optical power values. 
     
     
       9. The apparatus of  claim 1  further comprising at least one scanning mirror to reflect light produced by the laser diode in a raster pattern. 
     
     
       10. An apparatus comprising:
 a plurality of basis function circuits to receive a desired optical power value and produce basis function outputs as a function of the desired optical power value; 
 a plurality of multipliers to produce weighted basis function outputs from weight values and the basis function outputs; 
 a summer to sum the weighted basis function outputs to produce a compensation value to be applied to the desired optical power value; and 
 a circuit to adaptively modify the weight values from past weight values, past desired optical power values, and past measured optical power values. 
 
     
     
       11. The apparatus of  claim 10  further comprising a delay stage to delay the desired optical power value, wherein the plurality of basis function circuits produce the basis function outputs from delayed desired optical power values. 
     
     
       12. The apparatus of  claim 10  further comprising a delay stage to delay the desired optical power value, wherein the plurality of basis function circuits produce the basis function outputs from a combination of desired optical power values and delayed desired optical power values. 
     
     
       13. The apparatus of  claim 10  wherein the circuit to adaptively modify the weight values comprises a processor and a non-transitory storage medium encoded with instructions that when executed by the processor result in modifying the weight values. 
     
     
       14. The apparatus of  claim 13  wherein the processing system calculates the weights as a function of past basis function outputs, past measured optical powers, and past desired optical powers. 
     
     
       15. A method comprising:
 receiving a value representing a desired optical power; 
 determining a compensation value as a sum of weighted basis functions of the value representing the desired optical power; 
 adding the compensation value to the value representing the desired optical power to produce a value representing a compensated desired optical power; 
 determining a drive value from the compensated desired optical power and an inverse laser model; and 
 driving a laser diode with a current corresponding to the drive value. 
 
     
     
       16. The method of  claim 15  wherein determining a compensation value comprises determining a compensation value as a sum of weighted basis functions of the value representing the desired optical power and past values representing desired optical powers. 
     
     
       17. The method of  claim 15  further comprising summing past basis function output values. 
     
     
       18. The method of  claim 17  further comprising measuring optical powers produced by the laser diode. 
     
     
       19. The method of  claim 18  further comprising summing past measured optical powers produced by the laser diode. 
     
     
       20. The method of  claim 19  further comprising determining new weights as a function of the past basis function outputs, the past measured optical powers, and the past desired optical powers.